Transcriptional mapping of bacteriophage T4 messenger RNAs will be carried out using several techniques. Hybridization to cloned restriction fragments of T4DNA will be used to reveal the size of pulse-labelled transcripts. S1 nuclease digestion of DNA-RNA hybrids using plasmid DNA containing known T4 genes will be used to identify the initiation region of mRNAs from known T4 genes. The chemical and functional stability of T4 late messages will be compared, and together with data on the rate of transcription of the same late genes, the effect of post-transcriptional factors on T4 late protein synthesis can be assessed.